There is a blog post from Joel on the Stack Exchange blog around the time of WebApps' Area 51 Graduation:
A while ago, I wrote:
“Individually-branded sites felt more authentic and trustworthy. We thought that letting every Stack Exchange site have its own domain name, visual identity, logo, and brand would help the community feel more coherent. After all, nobody wants to say that they live in Housing Block 2938TC.”
Well, funny thing… that didn’t quite work out the way I expected... mostly because nobody could think of any good domain names. Believe it or not, “NothingToInstall” was one of the better suggestions. Ack.
This reasoning was used as justification for reverting "Nothing to Install" to "Web Applications", and maintaining graduating sites as "topic.stackexchange.com".